In order to extract a material heaped on a floor or in a bin, it is known to provide, in the center of the floor or bin, an orifice communicating with an evacuation pipe placed at a lower level, and to install above the plane of this floor or bin bottom an evacuation system or "extractor" composed of a worm parallel to this floor or bottom and provided both with rotational movement about its own axis and rotational movement about a vertical axis. The effect of this worm is to gather material from the various lower zones of the storage floor or bin and gradually to bring this material to the evacuation orifice.
Although they are simple of themselves, these extraction systems raise two problems: clogging of the material in the evacuation orifice, and maintenance of the worm-drive mechanism.
To resolve the clogging problem, it has been suggested that, above the evacuation orifice and the drive mechanism, a protective cap be provided, usually conical in shape, enabling the material to form a natural heap all around the evacuation orifice. To resolve the problem of maintaining the mechanism, it has been suggested to support the mechanism and the cap by a hollow beam traversing the storage floor or the bin and providing free access to the mechanism.
Systems improved in this way, however, have the disadvantage of being economically feasible only for storage floors or bins with diameters of no more than about ten meters. Above this dimension, it is necessary to use beams which, if they are to be of sufficient mechanical strength, must be of such dimensions and weight that they are very difficult to maintain and are excessive in cost.
Thus, to date there have been no evacuation devices which combine lightness, economy, and effectiveness.